This invention relates to polymerizable compositions for the organic coating of metallic materials, to a process for their production and to a process for the coil coating of steel.
In the metal-processing industry, particularly in the assembly of motor vehicles and in machine construction, the metallic components of the products have to be protected against corrosion. At present, the metal plates are first coated with corrosion-inhibiting oils at the rolling mill and are optionally coated with drawing oils before forming and die-stamping. In vehicle assembly or in machine construction, the metal plates shaped for bodywork, bodywork parts or machinery are stamped out and are formed by deep drawing using the drawing oils mentioned above. This is generally followed by assembly by welding and/or flanging and/or bonding andxe2x80x94after complicated cleaning stepsxe2x80x94by a corrosion-inhibiting surface pretreatment. This corrosion-inhibiting surface pretreatment consists of phosphating and, generally, chromating, after which the first coating of paint is applied to the parts in question, often by electrodeposition. This initial electrodeposition step is generally followed by the application of several other layers of paint, particularly in the case of car bodies.
Recently, so-called coil coating processes have been increasingly used for the pretreatment of steel plates because coil coating simplifies many process steps. In coil coating, steel strip is continuously pretreated and, optionally, organically coated at the rolling mill. Coil coating can save considerably on energy. In addition, it uses fewer chemicals and less fresh water and reduces the accumulation of wastewater. Nowadays, a range of steel strip surfaces, for example hot-dip-galvanized, electrolytically galvanized, galvannealed, aluminized surfaces, is available for improving the protection of the steel plates against corrosion. The steel strip thus treated is protected against corrosion immediately after production. Key standards for the quality of the vehicles or machinery produced from these steel plates are good paint adhesion and effective protection against corrosion. In conventional pretreatment processes based on coil coating, the use of chromium-containing products has hitherto been essential for increasing corrosion prevention and paint adhesion. Thus, even where such processes as alkaline passivation and the zinc or iron phosphating of steel or galvanized steel strip are used, after-passivation products containing chromic acid have to be used in order to obtain the required protection against corrosion. The wastewaters which accumulate where these processes are used have to be treated and disposed of at considerable cost. In addition, special safety measures have to be taken for handling chemicals containing chromium(VI) in the production of the pretreatment products, their transportation and their use.
A first step towards cleaner wastewater was the development of products based on chromic acid which were applied by a new process where no rinsing baths were used. This process came to be known as the no-rinse process.
Chromium-free no-rinse pretreatment processes have recently been developed on the basis of aqueous acidic solutions of fluoride-containing titanium salts and organic polymers as principal components. In addition, metal ions, such as manganese, cobalt, nickel or zinc, and phosphate and silicates may also be present. The organic constituents used are chelating polymers, such as polyacrylates or polyvinyl phenol derivatives. A key function of these chelating polymers is to form with the metal ions complexes which are stable in the aqueous phase. These chromium-free systems are described, for example, in EP 178020, in EP 469034 and in EP 555383. Although the processes just mentioned enable chromium-free systems to be used without any adverse effect on corrosion prevention, their disadvantage is that these chromium-free systems can only be applied from strongly acidic solutions and require a very high concentration of fluoride ions.
Accordingly, there is a need to develop pretreatment chemicals and pretreatment processes for chromium-free pretreatment which enable effective corrosion control to be obtained without having to use strong acids or high fluoride concentrations. The various constituents should preferably be uniformly distributed throughout the composition in order to avoid separation during production, transportation, storage and use.
The solution provided by the present invention is defined in the claims and essentially comprises providing polymerizable compositions for the organic coating of metallic materials which contain at least one titanium, manganese and/or zirconium salt of an olefinically unsaturated polymerizable carboxylic acid, at least one other olefinically unsaturated comonomer and at least one radical polymerization initiator in a non-aqueous system.
The present invention also relates to a process for the production of the organic coating compositions mentioned above and to a process for the coil coating of steel strip.
Suitable olefinically unsaturated polymerizable carboxylic acids for producing the corresponding titanium, manganese or zirconium salts are any homopolymerizable and/or copolymerizable olefinically unsaturated carboxylic acids known perse, namely: acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, maleic acid semiesters, i.e. esters of maleic acid in which a carboxyl group is esterified with an alkyl group, fumaric acid or fumaric acid semiesters, reactive carboxyfunctional macromonomers or mixtures of the acids mentioned above. xe2x80x9cMacromonomersxe2x80x9d in the context of the present invention are understood to be oligomers or polymers containing functional terminal groups through which the polymerization reactions can take place. In principle, therefore, they are macromolecular monomers. In general, these macromonomers contain unsaturated C,C-bonds, i.e. vinyl, acryl, methacryl or styryl groups, as terminal groups.
The composition may additionally contain other copolymerizable comonomers, such as hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, C1xe2x80x948 alkyl acrylates or methacrylates and one or more of the olefinically unsaturated carboxylic acids mentioned above in free form, i.e. not in salt form. The hydroxyfunctional comonomers mentioned above may even be partly replaced by polyethylene glycol semiesters of the olefinically unsaturated polymerizable carboxylic acids.
In addition, the compositions according to the invention contain at least one initiator for initiating the radical polymerization. The coating is cured by thermal polymerization, photopolymerization and/or electron-beam polymerization. Photopolymerization is particularly preferred so that the coating compositions suitable for photopolymerization contain a photoinitiator. Various commercially available photoinitiators are suitable. The compositions contain photoinitiators in quantities of 0.1 to 20% by weight, preferably in quantities of 2 to 12% by weight and more preferably in quantities of 3 to 8% by weight.
Examples of suitable photoinitiators are benzoins, benzoin alkyl ethers, benzil ketals such as, for example, benzil dimethyl ketal, acetophenone derivatives, for example dialkyl acetophenones, or even dichloro- and trichloroacetophenones. Examples of commercially available photoinitiators of the above-mentioned type are Irgacure 651 or 907 (Ciba Geigy).
In addition to or instead of the photoinitiators mentioned above, the compositions according to the invention may also contain free radical initiators of the peroxide type, for example benzyl peroxide, or azo compounds, for example 2,2xe2x80x2-azo-bis-isobutyronitrile. In cases where the polymerization is carried out solely as a thermal polymerization, the compositions do not of course contain a photoinitiator.
The polymerizable compositions according to the invention contain such quantities of the titanium, manganese and/or zirconium salt of the olefinically unsaturated carboxylic acid that the coating has a metal content of 0.5 to 15% and preferably 1 to 8%, up to 90% by weight of one or more of the comonomers and between 2 and 10% of the initiator.
Where alkyl acrylates and/or alkyl methacrylates are used as comonomers, the relatively long-chain alkyl compounds in particular should be present in small amounts because otherwise the compositions will not remain homogeneous in the long term. In addition, compositions such as these give poorer protection against corrosion. In general, the alkyl acrylates, for example 2-ethylhexyl acrylate in particular, should make up significantly less than 50% of the carboxyfunctional and/or hydroxyfunctional comonomers.
Polymerizable compositions of titanium, manganese or zirconium salts of acrylic or methacrylic acid with other acrylate or methacrylate comonomers are already known in principle. Thus, DE-A-2943566 describes a process for the production of an ionically crosslinked acrylic resin in which the monomer mixture contains 0.1 to 50% by weight of at least one metal salt of an unsaturated polymerizable carboxylic acid. These compositions are thermally polymerized as such. According to the document in question, the polymers obtained have an increased softening temperature and improved resistance to thermal decomposition by comparison with uncrosslinked polymers.
WO 89101952 describes a colorless transparent polymer material, preferably for optical purposes, produced by radical polymerization of a monomer mixture which contains from 20 to 40% by weight of a zirconium (IV)/acrylate compound.
EP-A-518609 describes a surface-textured film which is applied to a polymeric substrate and which consists of a hardened coating. Comonomer compositions which contain inter alia metal acrylates (zine diacrylate is mentioned in particular) and which are photopolymerized are proposed for these solvent-containing coatings.
None of the documents cited above discloses polymerizable compositions for the organic coil coating of metallic materials of the type according to the invention.
The compositions according to the invention may also contain additional additives, for example lubricants, in order to facilitate the subsequent forming of the correspondingly coated steel materials. However, it is a crucial requirement in this regard that the additives in question should not affect the crosslinking of the monomers.
The polymerizable organic medium is so polar that the salt-form metal components are able to react on the steel surface. It is assumed that this reaction of the titanium, manganese and/or zirconium salts on the steel surface or on the galvanized steel surface is responsible for corrosion control. Accordingly, it is all the more important that the compositions according to the invention enable a very large amount of corrosion-inhibiting metal to be introduced with little ballast, i.e. components which do not make any contribution towards corrosion control. At the same time, the metal complexing agent together with the metal becomes the binder component of the coating of the steel surface.
The compositions according to the invention may be produced in two different ways. Either the titanium, manganese and/or zirconium salts of the olefinically unsaturated polymerizable carboxylic acid are produced in a separate step and then dissolved in the remaining comonomers, followed by addition of the initiator(s).
In one particularly preferred process, the titanium, manganese and/or zirconium salts of the unsaturated carboxylic acid are produced in situ, i.e. the corresponding carboxylic acids are initially introduced and the corresponding metal oxides, metal hydroxides, metal alcoholates, metal carbonates and/or xcex2-dicarbonyl compounds of the metals are dissolved in the optionally heated carboxylic acid(s). The alcohols or xcex2-dicarbonyl compounds released may optionally be removed from the composition by distillation, optionally in vacuo. However, they may also remain in the composition.
Although the compositions according to the invention are substantially free from organic solvents, this does not mean that small quantities of alcohols, xcex2-dicarbonyl compounds or other volatile organic constituents from the production process cannot be present in small quantities in the composition.
The compositions according to the invention are normally applied to the steel plate in a sufficient quantity to obtain layer thicknesses of 0.5 to 10 xcexcm and preferably 1 to 3 xcexcm.